The manufacture of carbon-carbon products, in particular aircraft brakes, involves the deposition of carbon from the decomposition of hydrocarbon gases at high temperatures (e.g., about 1000° C.) and low pressures (e.g., about 0.01 atm absolute). Typically, the hydrocarbon gas is primarily methane. To enhance deposition rates, however, small amounts of heavier hydrocarbon gases are commonly included in the process.
The deposition of carbon in the manufacture of carbon-carbon composites is generally accomplished by Chemical Vapor Infiltration (CVI) or Chemical Vapor Deposition (CVD). For the purposes of the present invention, those processes may be regarded as interchangeable. Gas phase intermediates are formed from methane in these processes. Those skilled in the art generally agree that the gas phase intermediates formed from methane are acetylene, benzene, and naphthalene, e.g., by the reaction6 CH4→C6H6 (i.e., benzene) and 12 H2 Kinetic studies indicate that acetylene is formed first, then acetylene molecules combine to form benzene, and finally naphthalene is formed. The cyclic hydrocarbon molecules, benzene and naphthalene, are the precursors for carbon deposition. In the effluent gas, they act as tar formers. They continue to react and grow into higher molecular weight species that deposit on surfaces in the downstream gas processing equipment. Such tars represent a great expense in terms of vacuum pump maintenance, hazardous waste disposal, and so on. Tar traps have proven to be inefficient, and in any case do not solve the hazardous waste disposal problem. Moreover, even when tar traps are employed, there are still enough heavy hydrocarbons in the effluent gas to cause problems with pumps and combustion turbines, etc.
PRIOR ART. U.S. Pat. No. 6,270,741 B1 refers to a conventional method for producing iron carbide, in which fine-sized iron ore is charged into a fluidized bed reactor and reacted with a gas mixture that includes a reducing gas such as hydrogen and a carburizing gas such as methane. This reduces and carburizes iron oxides such as hematite (Fe2O3) and magnetite (Fe3O4) in a single process. For example, 3Fe2O3+5 H2+2CH4→2Fe3C+9 H2O. U.S. Pat. No. 6,328,946 B1 discloses a two-step process for producing iron carbide. In the first step, a feed material containing iron materials is contacted with a gas that contains hydrogen to produce an intermediate product containing metallic iron. In the second step, the intermediate product is contacted with a gas that includes (a) carbon monoxide and/or carbon dioxide and (b) hydrogen and optionally (c) methane. U.S. Pat. No. 6,428,763 B1 discloses a two-step process for producing iron carbide from an iron oxide-containing feed material. In the first step, a feed material containing iron oxide is converted to an intermediate product by contact with a reducing gas, and in the second step (carburization) step, the metallic iron intermediate is converted into an iron carbide product. The carburizing gas preferably contains no more than about 5 mole-% carbon dioxide, no more than about 15 mole-% methane, no more than about 10 mole-% water vapor, and no more than about 10 mole-% inert gases.